(1) Field of the Invention
The present invention relates to heavy duty pneumatic tires. More particularly, the invention relates to a technique for improving durability of bead portions in heavy duty pneumatic radial tires using cords of aromatic polyamide fibers as carcass ply cords arranged substantially at right angles with respect to the tire circumferential direction.
(2) Related Art Statement
When cords of aromatic polyamide fibers are used in a carcass ply of a pneumatic radial tire, various merits can be obtained. That is, a weight of the tire can be reduced, as compared with a tire using steel cords, to thereby reduce fuel consumption of a vehicle. However, in order to maintain the same carcass strength as in the use of steel cords, it is necessary to increase a diameter of the fiber cords, or narrow an interval between the cords, or increase the diameter of the cords and at the same time narrow the cord interval. As a result, there exists a problem in that a stress concentrates near end portions of the cords so that cracks are likely to occur at the cord end portions.
Therefore, when aromatic polyamide fiber cords are used, bead durability which is evaluated based on a running distance by which a bead undergoes trouble in a drum test is reduced by about 30% than that in the use of steel cords in the same bead structure. Thus, a technique for improving the bead durability has been required.
As a countermeasure for solving a problem of a reduced bead durability which is caused by earlier cracking at ply cord end portions as seen when the aromatic polyamide fiber cords are used in a carcass ply, for instance, Japanese patent application Laid-open No. 58-89,405 proposed a technique by which a deflection-deforming at end portions of turn-up portions of a carcass ply during running is prevented by arranging the end portions of the turn-up portions of the carcass ply lower than a rim flange, and an effect is actually obtained. However, in pneumatic tires, such as heavy duty pneumatic tires of a 15.degree. taper rim type, in which a rim flange is low, when end portions of turn-up portions of a carcass ply are located lower than the rim flange, the end portion of the turn-up portion of the carcass ply may be pulled out from a bead portion due to tire internal pressure, or a great separation occurs at the ply end at an earlier stage due to a stress concentration thereon. Consequently, there is a problem in that sufficient bead durability cannot be ensured.